The present invention generally relates to methods for fabricating a semiconductor device, and in particular relates to methods for fabricating a semiconductor device in which thermal processing is performed using infrared rays.
In fabricating a semiconductor device on a silicon substrate wafer, dopants have to be introduced into the wafer by, for example, ion implantation, vapor phase growth or solid diffusion, and thermal diffusion needs to be caused by conducting high-temperature annealing. In such thermal processing for the fabrication of a semiconductor device, it is essential that appropriate processing conditions be set in order to perform suitable thermal processing on a wafer.
Typical wafer thermal processing that has been conventionally employed is rapid thermal processing (RTP). This thermal processing utilizes light irradiation whereby the wafer can be processed at high temperature within a short period of time. In a general RTP apparatus, a tungsten halogen lamp is used to irradiate a wafer with light having a wavelength of 0.4 μm to 5.0 μm and allow the wafer to absorb the light, thereby heating the wafer. Furthermore, a pyrometer (radiation pyrometer) is used to detect infrared irradiation emitted from the bottom surface of the wafer and measure the wafer temperature based on the intensity of the infrared irradiation. Hereinafter, a conventional RTP process will be described with reference to the drawings.
FIG. 6 is a schematic diagram of an RTP apparatus of the type that has been conventionally used.
The RTP apparatus includes: a tungsten halogen lamp 101 for heating a semiconductor substrate 102; a pyrometer 103 for measuring the temperature of the semiconductor substrate 102; a controller 104 for controlling the power of the tungsten halogen lamp 101; and a driver 105 for driving the tungsten halogen lamp 101. The pyrometer 103 detects an infrared ray emitted from the wafer to measure the wafer temperature.
It is to be noted that, in order to increase the temperature controllability over the wafer surface, the RTP apparatus is provided with a plurality of the pyrometers 103, thus carrying out temperature measurements at a plurality of points of the semiconductor substrate 102. The results of the temperature measurements are sent to the controller 104 of the RTP apparatus. Next, the controller 104 sends information on the power of the tungsten halogen lamp 101 (a set point for the lamp) to the driver 105 based on the measurement results, and then the tungsten halogen lamp 101 driven by the driver 105 carries out irradiation in accordance with the lamp power information.
FIG. 7 is a graph showing an exemplary conventional temperature profile during processing carried out by the RTP apparatus. In the graph, the abscissa represents time while the ordinate represents temperature, and readings of five pyrometers are shown.
As shown in FIG. 7, the conventional RTP process includes: the step of preheating a wafer (herein called a “preheating step”); the step of stabilizing the wafer temperature (herein called a “stabilizing step”); the step of increasing the wafer temperature (herein called a “temperature increasing step”); the step of thermally processing the wafer at a processing temperature (herein called a “processing temperature step”); and the step of reducing the wafer temperature (herein called a “temperature reducing step”).
In the example shown in the graph, the preheating step is first performed for about 10 seconds. Thereafter, the wafer is heated to a temperature range in which the controllability of the pyrometers is ensured, and then the temperature of the wafer surface is stabilized. This stabilizing step is performed for about 15 seconds. Subsequently, when the temperature at each point of the wafer has reached a predetermined value, the wafer is increased in temperature to the processing temperature, and thermally processed at the processing temperature for about 30 seconds. Then, the lamp power is shut off to reduce the wafer temperature.
It is to be noted that the wafer surface temperature is stabilized in the temperature range lower than the processing temperature in order to reduce variations in the wafer surface temperature during the processing temperature step.
The temperature profile shown in the graph is just an exemplary one obtained in the RTP process; therefore, the processing temperature and processing time differ depending on the design of the semiconductor device to be fabricated.
As can be seen from the temperature profile shown in FIG. 7, in the conventional RTP process, variations in the wafer surface temperature are observed during the processing temperature step. In order to meet standard requirements for the performance of the product, the RTP process has to be performed at the processing temperature with an accuracy of ±2.5° C. achieved in the temperature profile. However, in the conventional thermal processing method shown in FIG. 7, this condition is not necessarily satisfied.
In the latest semiconductor device fabrication, the period of time required for a processing temperature step in an RTP process may be 10 seconds or shorter, and a spike annealing may be performed in which a wafer is momentarily subjected to thermal processing at the highest possible temperature. In those cases, the above-described nonuniform temperature of applied heat might cause critical problems.
In order to prevent such problems while increasing temperature controllability in a low temperature range, a lamp light source for irradiating a wafer with visible light is used, or a pyrometer or a processing chamber of an RTP apparatus is changed in structure. However, such measures not only make the apparatus expensive but also make it difficult to cope with a wide thermal processing temperature range. Accordingly, the above-mentioned measures are not sufficiently effective.